The present invention relates generally to a feed forward power amplifier, and specifically to improvements in distortion cancellation and improvements in carrier cancellation and efficiency.
To provide high quality voice communication and handle more calls, telecommunication operators use digital modulation technology and multi-channel carriers. Examples of such technologies include CDMA, TDMA, w-CDMA, and EDGE-GSM. A linear power amplifier is needed to transmit digital/multi-carrier waveforms without adding noise, distortions, and adjacent channel interference. Typical RF power amplifiers used for a base station generate xe2x88x9230 dBc distortion levels. A digital/multi-carrier system, however, requires a power amplifier with xe2x88x9245 dBc for CDMA, xe2x88x9260 dBc for TDMA, and xe2x88x9270 dBc for GSM.
Several linearization techniques have been used to linearize or improve the traditional class AB power amplifier distortion performance. Such techniques include RF feedback, Cartesian feedback, analog pre-distortion, digital pre-distortion and feed forward. The RF feedback and Cartesian feedback techniques improve inter-modulation distortion (xe2x80x9cIMDxe2x80x9d) by 10 db but both techniques are only useful for narrowband. Analog pre-distortion improves IMD up to 8-9 db but is not stable over certain temperatures. Digital pre-distortion uses lookup table A/D and D/A converters to distort the input signal. This technique requires very fast A/D and D/A converters. Successful results have not been reported for work involving this technique.
The feed forward technique improves IMD by almost 30 db with reasonable bandwidth. The feed forward technique is widely recognized and is used by several power amplifier companies. This technique was disclosed in 1930 by H. Black, U.S. Pat. No. 1,686,792. The feed forward technique involves splitting an input signal into two paths, which process along two loops, the first loop producing a distortion component by combining an out of phase signal segment with an amplified signal containing distortion, and the second loop removing the distortion from the amplified signal by using an out of phase distortion component.
Improvements have been disclosed to the feed forward system. Improvements to the first loop have included adding control circuitry to monitor the carrier residue and adjust the gain and phase accordingly to minimize the carrier residue. Improvements to the second loop have included pilot tone injection at the input of a main amplifier which behaves as a distortion. So far, most improvements only aim to solve loop stability. The feed forward amplifier in the existing marketplace, however, does not meet the stringent Global System for Mobile communications (xe2x80x9cGSMxe2x80x9d) requirements. A dual loop feed forward may meet the GSM requirements, but this approach is very costly and has low efficiency due to using two error amplifiers and two output delay lines. Several power amplifier manufacturers have abandoned this approach.
U.S. Pat. No. 5,412,342 to Sakamoto, et al., for xe2x80x9cPower Amplifier Device Comprising a Plurality of Feedforward Distortion Compensating Circuits in Parallel,xe2x80x9d discloses using random phase cancellation in parallel feed-forward circuits, and more particularly, discloses a power amplifier device comprising a feed forward distortion compensating circuit comprising N complete feed forward circuits (N being an integer not less than two) that are connected in parallel and are supplied with input radio frequency signals through an N-divider. Component outputs of the feed forward circuits are combined in phase as the output signal by an N-combiner.
The use of N complete feed-forward amplifier circuits, however, is costly. Sakamoto does not disclose random phase cancellation within a stage of a single feed-forward circuit, which would be a less costly alternative.
U.S. Pat. No. 6,037,837 to Miyaji, et al., for xe2x80x9cFeed Forward Amplifierxe2x80x9d discloses a feed forward power amplifier which cancels distortion by dividing an input signal into two divided signals and employs a vector adjuster to adjust the phase of the distortion components. The use of N power amplifiers connected in parallel coupled with a vector adjuster, is disclosed, but random phase cancellation within a stage of a single feed-forward circuit is not disclosed. The need to employ N power amplifiers and vector adjuster circuitry makes this approach more costly than random phase cancellation within a stage of a single feed-forward circuit.
What is needed is a cost efficient feed forward power amplifier to transmit digital/multi-carrier waveforms with minimum distortion, preferably using random phase cancellation within a stage of a single feed-forward circuit.
The present invention meets this need by providing a cost efficient feed forward power amplifier for transmitting digital/multi-carrier waveforms with minimum distortion, using random phase cancellation within a stage of a single feed-forward circuit.
An N way carrier cancellation coupler is provided, N being greater than 1, for use in a feed forward power amplifier, comprising a first N way divider, having an input and N outputs, a second N way divider, having an input and N outputs, an N way combiner having N inputs and an output, N carrier cancellation couplers, each having a first input, a second input and an output, the first input of each of the N carrier cancellation couplers connected to one of the N outputs of the first N way divider, the second input of each of the N carrier cancellation couplers connected to one of the N outputs of the second N way divider, and the output of each of the N carrier cancellation couplers connected to one of the N inputs of the N way combiner.
An M way distortion cancellation coupler is also provided, M being greater than 1, for use in a feed forward power amplifier, comprising a first M way divider, having an input and M outputs, a second M way divider, having an input and M outputs, an M way combiner having M inputs and an output, M distortion cancellation couplers, each having a first input, a second input and an output, the first input of each of the M distortion cancellation couplers connected to one of the M outputs of the first M way divider, the second input of each of the M distortion cancellation couplers connected to one of the M outputs of the second M way divider, and the output of each of the M distortion cancellation couplers connected to one of the M inputs of the M way combiner.
An N way carrier cancellation coupler is described for use in the first loop stage of a feed forward power amplifier, comprising first divider means for dividing a signal N ways and outputting N divided signals, second divider means for dividing a delayed signal N ways and outputting N delayed signals, N carrier cancellation coupler means, each for receiving one of the N divided signals from the first divider means, and one of the N delayed signals from the second divider means, and producing one of N error signals from a coupled form of the received one of the N divided signals and received one of the N delayed signals; and signal combiner means for receiving each one of the N error signals from the N carrier cancellation coupler means and producing a combined error signal from a combined form of the N error signals.
An M way distortion cancellation coupler is described for use in the second loop stage of a feed forward power amplifier, comprising first divider means for dividing an amplified error signal M ways and outputting M divided error signals, second divider means for dividing a delayed signal with distortion M ways and outputting M divided signals, M distortion cancellation coupler means, each for receiving one of the M divided error signals from the first divider means, and one of the M divided signals from the second divider means, and producing one of M main signals from a coupled form of the received one of the M divided error signals and received one of the M divided signals; and signal combiner means for receiving each of the M main signals from each of the M distortion cancellation coupler means and producing an output signal from a combined form of the M main signals.
A method is described for reducing the carrier component of a signal in the first loop stage of a feed forward power amplifier, comprising the steps of (a) dividing the signal N ways, N being greater than 1, to produce N divided signals, (b) dividing a delayed form of the signal N ways to produce N delayed signals, (c) coupling one of the N divided signals and one of the N delayed signals to produce one of N error signals; (d) repeating step (c) until each one of the N divided signals has been coupled with one of the N delayed signals to produce one of N error signals; and (e) combining each of the N error signals to produce a combined error signal from a combined form of the N error signals.
A method for reducing the distortion component of a signal in the second loop stage of a feed forward power amplifier is also described comprising the steps of (a) dividing the distortion component of the signal M ways, M being greater than 1, to produce M divided error signals, (b) dividing a delayed form of the signal M ways to produce M divided signals, (c) coupling one of the M divided error signals and one of the M divided signals to produce one of M main signals; (d) repeating step (c) until each one of the M divided error signals has been coupled with one of the M divided signals to produce one of M main signals; and (e) combining each of the M main signals to produce an output signal from a combined form of the M main signals.
In another embodiment either of the methods so described further comprise the step of selecting the components used in coupling step (c) to complement one another during manufacture.
A power amplifier using the invention is described comprising a feed forward power amplifier having a first loop and a second loop; and an N way carrier cancellation coupler, N being greater than 1, having a first input, a second input, and an output, both the first input and second input connected to the first loop of the feed forward power amplifier, and the output connected to the second loop of the feed forward power amplifier.
The power amplifier with an N way carrier cancellation coupler may further comprise an M way distortion cancellation coupler, M being greater than 1, having a first input, a second input, and an output, both the first input and second input connected to the second loop of the feed forward power amplifier.
In another embodiment the power amplifier comprises a feed forward power amplifier having a first loop and a second loop; and an M way distortion cancellation coupler, M being greater than 1, having a first input, a second input, and an output, both the first input and second input connected to the second loop of the feed forward power amplifier.
In one embodiment of the invention a feed forward power amplifier is described comprising an input coupler for receiving an input signal and dividing it in to a main signal and a first signal; a main amplifier connected to the input coupler for receiving the main signal and outputting a distorted amplified signal; a first delay circuit connected to the input coupler for receiving the first signal and outputting a first delayed signal; a sample coupler connected to the main amplifier for receiving the distorted amplified signal and dividing it into a first divided signal and a second divided signal; an attenuator connected to the sample coupler for receiving the second divided signal and outputting an attenuated signal with distortion; an N way carrier cancellation circuit connected to the attenuator for receiving the attenuated signal with distortion and the first delay circuit for receiving the first delayed signal, and outputting a combined error signal; an error amplifier connected to the N way carrier cancellation circuit for receiving the combined error signal and outputting an amplified error signal; a second delay circuit connected to the sample coupler for receiving the first divided signal and outputting a delayed signal with distortion; and an M way distortion cancellation circuit connected to the second delay circuit for receiving the delayed signal with distortion and the error amplifier for receiving the amplified error signal, and outputting the output signal; wherein either N is greater than 1 and M is 1, N is 1 and M is greater than 1, or N is greater than 1 and M is greater than 1.
In such embodiment either the N way carrier cancellation circuit, or the M way distortion cancellation circuit, or both the N way carrier cancellation circuit and the M way distortion cancellation circuit, comprises a plurality of cancellation couplers, two signal dividers and a signal combiner, the input of each of the plurality of cancellation couplers connected in parallel to each of the signal dividers, and the output of each of the plurality of cancellation couplers connected to the signal combiner.
A feed forward power amplifier with reduced distortion in an output signal having a first loop stage and a second loop stage is therefore described, wherein the reduction of carrier residue in the first loop stage, or the reduction of distortion in the second loop stage comprises random phase cancellation.
A feed forward power amplifier for reducing distortion in an output signal is disclosed comprising means for receiving an input signal and dividing it into a main signal and a first signal; means for amplifying the main signal and outputting a distorted amplified signal; means for processing the first signal to produce a first delayed signal; means for dividing the distorted amplified signal into a first divided signal and a second divided signal; means for attenuating the second divided signal to produce an attenuated signal with distortion; means for splitting N ways both the attenuated signal with distortion and the first delayed signal and combining the split signals into a combined error signal; means for amplifying the combined error signal to produce an amplified error signal; means for processing the first divided signal to produce a delayed signal with distortion; and means for splitting M ways both the delayed signal with distortion and the amplified error signal and combining to produce the output signal; wherein either N is greater than 1 and M is 1, N is 1 and M is greater than 1, or N is greater than 1 and M is greater than 1.
A method for reducing distortion in an output signal from a feed forward power amplifier is disclosed comprising the steps of receiving an input signal and dividing it into a main signal and a first signal; amplifying the main signal and outputting a distorted amplified signal; processing the first signal to produce a first delayed signal; dividing the distorted amplified signal into a first divided signal and a second divided signal; attenuating the second divided signal to produce an attenuated signal with distortion; splitting N ways both the attenuated signal with distortion and the first delayed signal and combining the split signals into a combined error signal; amplifying the combined error signal to produce an amplified error signal; processing the first divided signal to produce a delayed signal with distortion; and splitting M ways both the delayed signal with distortion and the amplified error signal and combining to produce an output signal; wherein either N is greater than 1 and M is 1, N is 1 and M is greater than 1, or N is greater than 1 and M is greater than 1.
In a preferred embodiment, the method so disclosed further comprises the step of selecting the components used in splitting N ways or splitting M ways to complement one another during manufacture.